How to erase any residual keyframes from a .blend file?
I got rid of it but using vers 2.77a.  Let's hope it works in your version...

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The problem was a grease pencil hangover and it's slot that was still active.

Procedure to rid yourself of that pesky keyframe...

Zoom in a bit to see the G/pencil drawing south of the grid.

Goto Edit mode and press A.  The GP points will all hilight

Delete them with X.  (points)

Press N to open the 3D viewport's properties and delete the Grease Pencil slot.

Untick the grease pencil's box.

The keyframe will still be there.  

Save the file as something else and the keyframe should disappear when the 3D widow updates.

If not exit Blender and reload.